Reliable range measurement systems are essential for estimating position of an object in motion. For example, traditional altimeter measurements determine the altitude of the platform above a fixed level by measuring air pressure (where air pressure decreases with an increase of altitude). A radar altimeter measures altitude by using the time for energy (that is, a radar pulse) to reflect from a target back to the object. Radar altimeters are a typical embodiment of a range measurement system. As a range measurement system, the radar altimeter component informs an operator (for example, an aircraft pilot) that an aircraft is flying too low or that terrain is rising to meet the aircraft.
Typical pulsed radar systems use one or more gates to measure the reflected energy within a prescribed time period. Each gate is considered to represent a window of time in which the system monitors the reflected energy (that is, the radar altimeter begins accumulating energy at the beginning of each gate and stops accumulating this energy at the end of each gate). A typical radar altimeter device may include three gates that are in use during a track mode, of which only one is used during a search mode. Once in the search mode, a range gate is positioned at a particular location for a specified time. If the target (for example, the terrain below the aircraft) is not detected at that gate position after waiting the specified time, the range gate moves out incrementally.
Functionality of current pulsed radar altimeter devices is limited with respect to the amount of time it takes the device to acquire the target, especially at higher altitudes of 15,240 m (50,000 ft) and above. For any decreases in search response times, the range measurement system is expected to maintain current performance criteria, including measurement sensitivity and probability of detection.